1. Field of the Invention
The present invention relates to a brake pressure maintaining apparatus for a vehicle including a normally-open-type electromagnetic valve which, even after a driver releases his or her foot from a brake pedal to thereby remove a braking operation, is closed so as to be able to maintain the brake fluid pressure of a wheel brake temporarily. And, the normally-open-type electromagnetic valve has the following relief function: that is, the brake fluid pressure to be maintained by the wheel brake is set according to the value of a current to be applied to an electromagnetic coil and, when the brake fluid pressure of the wheel brake is higher than the thus-set given value, the normally-open-type electromagnetic valve is temporarily opened to thereby lower the brake fluid pressure of the wheel brake down to the given value.
2. Description of the Related Art
Conventionally, in order to maintain the brake fluid pressure of the wheel brake temporarily even after removal of the brake operation, as a vehicle brake fluid brake maintaining apparatus provided with a normally-open-type electromagnetic valve, there are known the cited related inventions (JP-A-2000-190828 and JP-A-2001-225731). In the case of the normally-open-type electromagnetic valve used in these conventional related arts, in the interior of a housing which is disposed in a brake fluid flow passage between a master cylinder and a wheel brake, there is disposed a valve seat including a communication hole which is formed in the central portion of the valve seat so as to allow the master cylinder side and wheel brake side to communicate with each other; and, a spherical-shaped valve body is disposed so as to face the valve seat. The valve body, when the normally-open-type electromagnetic valve is not in operation, is normally energized by the energizing force of a return spring in a direction where it is moved apart from the valve seat, and thus the communication hole is held in the open state. Thanks to this, in the braking operation, the brake fluid pressure is transmitted from the master cylinder through the communication hole to the wheel brake, thereby being able to brake the wheels of the vehicle.
Also, when maintaining the brake fluid pressure even after removal of the braking operation, an electromagnetic force is applied to the normally-open-type electromagnetic valve, the valve body is energized against the energizing force of the return spring in a direction where it is seated on the valve seat, and thus the valve body is pressed against the valve seat, thereby closing the normally-open-type electromagnetic valve. Due to the closed state of the normally-open-type electromagnetic valve, in the braking operation, the return of the brake fluid pressure from the wheel brake to the master cylinder can be prevented, which makes it possible to maintain the brake fluid pressure of the wheel brakes temporarily. Thus, the sudden start of the vehicle in a descending slope and the backing movement of the vehicle in an ascending slope can be prevented, thereby allowing the vehicle to start smoothly
Also, in the brake fluid pressure maintaining operation due to the closed state of the normally-open-type electromagnetic valve, when the brake fluid pressure of the wheel brake is higher than necessary, the brake fluid pressure of the wheel brake is relieved to the master cylinder by a relief valve to thereby be able to restrict the dragging of the brake force in the time of the start of the vehicle. This conventional relief valve, normally, is closed by the energizing force of an energizing spring or the like, and can be opened against the energizing force only when the brake fluid pressure of the wheel brake is higher than the energizing force. And, in case where the normally-open-type electromagnetic valve is lowered down to a given value which can be determined by the energizing force of the energizing spring, the relief valve is closed again.
After closing of the relief valve, by returning the brake fluid pressure of the wheel brake to the master cylinder gradually through an orifice consisting of a minute recessed groove formed in a flow passage for the brake fluid pressure, not only can the vehicle be started smoothly but also the brake fluid pressure of the wheel brake can be lowered bit by bit down to the value of the brake fluid pressure that is indispensably necessary to help the start of the vehicle. By lowering the brake fluid pressure gradually in this manner, when the driver steps on an accelerator pedal, the backing movement of the vehicle in the ascending slope and the sudden start of the vehicle in the descending slope can be made hard to occur and also the dragging of the braking force can be restricted, thereby allowing the vehicle to start smoothly.
However, in the above related art, since the relief mechanism is structured in such a manner that the valve members and energizing spring are disposed in a narrow space within a movable core and the orifice is formed by working a recessed groove in the valve seat, the number of parts of the brake fluid pressure maintaining apparatus as well as the number of working and assembling steps thereof are large. Also, working of the recessed groove for use as an orifice requires high level of working accuracy and, at the same time, because the flow quantity of the brake fluid cannot be predicted, after manufacture of a product, the brake fluid must be actually made to flow in order to check the flow quantity of the brake fluid, which provides a limit on enhancement in the productivity. Also, according to the type of a vehicle, such as the weight of the vehicle on which the brake fluid pressure maintaining apparatus is to be mounted, the energizing force of the energizing spring of the relief valve and the depth of the orifice vary, which makes it necessary to set the specifications of a plurality of structures; that is, it is troublesome to manufacture the brake fluid pressure maintaining apparatus according to the above related art.